(31b) Molecular Dynamics Simulations of the Crystallization of Polyethylene As a Function of Chain Orientation

Both all-atom and united-atom molecular dynamics (MD) simulations are used to quantify the role of nematic ordering in the nucleation of both linear and short-chain-branched polyethylene (PE) when chains are at equilibrium and when they are stretched. The nematic coupling parameter that reflects the tendency of oriented chains to induce orientation on their neighbors in inferred from atomistic simulations obtained by imposing a tension on a fraction of the chains, and measuring induced nematic order on the other chains. By simulating isothermal nucleation and performing mean-first- passage time analysis for the growth of the largest nucleus in our simulations, we show that the nucleation rate of PE increases exponentially with increasing average nematic order of monomers and is strongly slowed by short-chain side branches. We also validate the predicted quiescent nucleation rates by computing the crystallization half-time at various temperatures using the crystal growth velocity, sampled from simulations of isothermal crystallization of free chains near crystalline slabs, and the Avrami equation. The predicted crystallization half-times are in agreement with experiments, suggesting our predicted nucleation rates help in developing a quantitative theory of flow-enhanced crystallization of polyethylene.